Filter material for fluids and method for producing a filter material

ABSTRACT

The invention relates to a filter material ( 2 ) for fluids, in particular hydraulic fluids, comprising a filter medium ( 4 ) having at least one layer, and a supporting structure ( 6 ) which is partially made of a plastic material and rests flatly in some surface areas on at least one side of the filter medium ( 4 ). Said filter material is characterized in that the filter medium ( 4 ) and the supporting structure ( 6 ) are connected to one another by laminating, covering and/or by a melting process.

The invention relates to a filter material for fluids, in particular for hydraulic fluids, comprising a filter medium having at least one layer and a support structure which is formed at least partially from a plastic material and which adjoins at least one side of the filter medium in surface regions, and a method for producing such a filter material.

Filter materials for producing interchangeable filter elements in hydraulic systems are known in diverse configurations and consist, for example, of a filter nonwoven, preferably of several layers, with a support structure on one or both sides (incident flow side or outflow side). A hydraulic fluid to be filtered flows through these types of filter elements, in part with a considerable pressure difference arising from the unfiltered side to the filtered side. In order to be able to withstand this pressure difference and also dynamic flow forces in the unfiltered material, the filter materials from which corresponding filter elements are produced have so-called support structures. Such support structures are subject to strong cyclic pressure loading in the operation of a filter element and are generally formed from metal fabrics, especially fabrics made of high-grade steel wires.

EP 1 436 062 B1 discloses a filter element for fluids, in particular hydraulic fluids, comprising a filter material of the aforementioned type. The filter material comprises a filter medium with a latticed support structure which supports the filter medium with respect to the flow direction through the filter element at least on the filtered side, with the support structure being produced from a plastic material and having electrically conductive elements. The support structure is made as a support fabric of metal filaments and plastic threads and rests flat on the filter medium. The filter medium and the flat-resting support fabric or the support structure are folded up into a star shape.

Another generic filter material for fluids and a method according to the preamble of claim 12 are disclosed in DE 200 13 839 U1. The known filter material is a textile composite which consists partially of an electrically conductive material and contains at least one support layer formed by threads which are arranged crosswise to one another, and at least one fiber support formed by a pinned non-woven fabric. The support layer or the support structure is made as a woven fabric or a non-crimp fabric, with some of the threads of the woven fabric being formed from an electrically conductive material, preferably metal. The other threads are formed from a plastic material in the same manner as the non-woven fabric which forms the filter medium. To carry out the needling process, the puncture depth is advantageously chosen such that the needles completely pierce the non-woven fabric. The puncture density is typically 800 punctures per cm². The joining of the two layers by means of needling is complex and consequently costly.

Based on this prior art, the object of the invention is to provide a filter material which can be produced or supplied easily and economically without limiting its functionality and stability.

This object is achieved according to the invention by a filter material for fluids having the features specified in claim 1 in its entirety and a method for producing a filter material for fluids according to claim 12.

Because the filter medium and the support structure are joined or are being joined to one another by means of lamination, coating, and/or by means of a melting method, a connection between the at least one layer of the filter medium and the support structure is easily formed, and both the strength of the support structure and also a good permanent connection between the filter medium and support structure are ensured. The improved and stiffer support leads to an improved differential pressure behavior of the filter material in a filter element and to an improved collapse stability. The individual layers of the filter material according to the invention which are joined flat to one another have much greater stability of shape than conventional materials, as a result of which distortion in one spatial direction is largely prevented.

Another advantage is the thin execution of the filter material which has been produced by lamination, coating, and/or melting. In particular, the filter material is thinner than the individual layers so that by forming additional folds a greater filter area can be formed in a filter element and consequently the available installation space is better used for filtration. The improved drainage function of the filter material layers which have been joined to one another according to the invention leads to a lower differential pressure and consequently to an improved operating behavior of a filter element which has been equipped with the filter material according to the invention.

In an advantageous configuration of the filter material according to the invention, adhesive spots which have been applied at least in individual areas in a spray application adhere to the filter medium and especially are provided in different sizes and/or in an irregular arrangement. Especially preferably, the adhesive spots and/or an additional coating provided on the filter medium can partially enclose at least some of the passage openings for fluid which are dictated by the support structure. In this way, the fluid permeability of the filter medium is not adversely affected, and consequently its filtration capacity is essentially preserved in spite of the applied support structure. A filter material configured in this way is used in particular for fine filter elements. The spray application of the adhesive on the one hand yields an adhesive bond that is uniformly strong over the area of the filter medium between the support structure and the filter medium, as well as an additional stiffening of the filter medium with minimum possible adverse effect on the fluid permeability of the passage openings covered by the filter medium.

In a preferred embodiment of the filter material according to the invention, the material of the support structure comprises polybutylene terephthalate (PBT) plastic, polypropylene (PP) plastic, and/or polyethylene (PE) plastic. These plastics are characterized by an average strength, a high stiffness, and hardness. Partially crystalline polyethylene terephthalates (PET) have thermal boundaries of use of from −20° C. to about +100° C., briefly up to +200° C. They are resistant to dilute acids, aliphatic and aromatic hydrocarbons, oils, fats, esters, and alcohols. Compared to polyethylene terephthalate, polybutylene terephthalate has a somewhat lower strength; its boundaries of use are about −40° C. to +250° C. Isotactic polypropylene can be used up to about 150° C. and is chemically very stable.

In another preferred embodiment of the filter material according to the invention, the support structure is made as a lattice or a woven fabric. A latticed or grid-shaped arrangement of thread elements yields a uniform stability of shape and filtration characteristics which are homogeneous over the area of the filter material.

Preferably, the thickness of the thread elements which form the lattice or woven fabric varies over the area of the filter material, especially when the thread elements in the weft direction have a thickness that is different from that of the thread elements in the warp direction. Execution as a lattice or woven fabric yields good strength properties and good fluid permeability; by using thread elements of different thicknesses, the passage area through which the fluid can flow is further enlarged.

The thickness of the thread elements in the weft direction is preferably 250 μm and in the warp direction preferably 200 μm. Furthermore, it is advantageous for the mesh size of the lattice or of the woven fabric to be 850 μm×1200 μm. With this dimensioning, the supporting lattice or support fabric formed by the thread elements has good strength properties and moreover a maximum possible fluid passage area, in other words, high fluid permeability. Depending on the mesh size, the support structure effects coarse filtration for particles whose dimensions exceed the mesh size.

Moreover, at least one electrically conductive line element can be worked into the support structure, with the at least one electrically conductive line element preferably being made metal-free and/or containing carbon. Especially preferably, the electrically conductive line element is made as a bicomponent fiber with a carbon-coated plastic fiber.

The filter medium which effects the actual filtration is made as a nonwoven, preferably as a plastic nonwoven, especially preferably as a polyester nonwoven.

The material of the filter medium comprises preferably polybutylene terephthalate (PBT) plastic, polypropylene (PP) plastic, and/or polyethylene (PE) plastic. It is furthermore conceivable for the laminated nonwoven to be made conductive in order to improve the electrostatic properties of the filter material.

It is furthermore advantageous if the filter medium and the support structure are joined to one another like a blanket. This prevents the filter medium and support structure from lying apart from one another, which is undesirable and adversely affects the supporting and filtration action, and avoids spacing between the filter medium and support structure in surface areas which are not joined to one another when the filter material is used properly, for example, in pleat-like folding-up.

In one preferred version of the method according to the invention, the support structure is laminated onto the filter medium as a coating. This creates a permanently strong connection of the filter material layers; this allows higher strength with a lower material thickness of the support structure. The filter material which has been formed in this way with a support structure applied on one or both sides to a filter medium, especially a filter nonwoven, can be further processed into filter elements which have very good stability of shape even under changing pressure loads. The filter medium and the support structures can be produced first as separate material layers and then joined to one another. The support structure can, however, also be made in one pass and joined to the filter medium. The pressure for fixing the support structure on the filter medium can be applied, for example, via a roller.

The support structure can be applied to the filter medium in an immersion method. Here, for example, a mask is placed on the filter medium and the latter is then immersed in a bath with plastic-containing material for the support structure. The open spaces of the mask corresponding to the support structure to be formed are filled with plastic material which cures and compacts after immersion and in this way forms the support structure in direct connection to the filter medium. To cure and compact the support structure, heating or cooling can additionally be carried out. Excess material can be removed with a doctor blade.

Furthermore, the support structure can be applied to the filter medium in a doctoring method. With the doctoring method, the elements which form the support structure, such as the thread elements of a lattice or a woven fabric, are applied to the filter medium in a preferably uniform pattern. Alternatively, the prefabricated support structure, especially partially compacted and cured, can be placed flat on the filter medium by means of a carrier roller and can be joined to the filter medium.

In another preferred version of the method according to the invention, the support structure is calender-coated onto the filter medium in a cementing process. In doing so, depending on the requirements imposed on the filter material, support structures of the most varied form and execution can be applied to a filter medium. In particular, the execution and application of the support structure can be done in one process step; i.e., a plastic material which has adhesive properties is applied to the filter medium corresponding to the desired support structure and then cures. Depending on the requirements, support structures with certain supporting and filtration properties can be formed by the type and the alignment of the plastic threads or fibers and by the choice of the plastic material.

It is furthermore advantageous if an adhesive is applied to the filter medium in a carrier application. In doing so, the adhesive is not applied directly, for example, by a nozzle, to the filter medium, but first to a carrier, such as a carrier roller, and then to the filter medium. This yields the advantage that the carrier structure is formed first on a carrier, partially cured, and can be checked for faults before it is cemented or calender-coated onto the filter medium.

Advantageously, the adhesive is applied to the filter medium in a spray application. As the adhesive is being sprayed on, droplet-like adhesive spots are formed on the filter medium. The adhesive spots create an adhesive bond between the filter medium and the support structure upon contact with the support structure. The adhesive spots or adhesive droplets which are not used for cementing and which are located in passage regions of the support structure lead to a reinforcement of the nonwoven-like filter medium, with the fluid passage and the three-dimensional deformability of the filter medium being almost unaffected. Alternatively or in addition to the adhesive spots of adhesive, an additional coating can be applied to the filter medium, with the adhesive spots and/or the coating advantageously being applied to the filter medium such that at least some of the passage openings for fluid which are dictated by the support structure are at least partially enclosed or covered.

It is furthermore advantageous if an adhesive layer is applied to the filter medium and/or the support structure. In doing so, the entire contact surface between the support structure and the filter medium is used as a joining or adhesive surface, as a result of which a good connection between the two material layers is ensured. This is especially important when the support structure has thin thread elements compared to the passage regions.

Advantageously, a self-adherent adhesive, especially a hot-melt adhesive, which joins the filter medium to the support structure, is used. In doing so, additional heating of the adhesive to form an adhesive bond can be omitted so that the method according to the invention can be easily carried out.

In another preferred version of the method according to the invention, the support structure is applied to the filter medium in a thermal melting method, especially in an ultrasonic method. In ultrasonic bonding, mechanical vibrations are transferred to the plastic parts under pressure. Molecular friction and interface friction generate heat that allows the damping coefficient of the material to increase and allows the plastic to soften locally. This reaction accelerates by itself, since a greater proportion of the vibration energy is converted into heat due to the increase in the damping factor of the plasticized material. After completion of sonic irradiation, a short cooling phase under the still prevailing joining pressure is necessary in order to uniformly compact the previously plasticized material. Afterwards, the material layers which have now been joined using ultrasonic energy can be further processed. The rapid and controlled melting of the materials is achieved by suitable bonding geometries, such as tips, edges, or rotations in the joining zones or on sonotrons or an anvil. The parts are thermally loaded only to a minor degree due to the low energy required for ultrasonic bonding. The basic properties of the materials to be bonded are not altered by the use of ultrasound. A high process speed and the simultaneous execution of working steps such as ultrasonic bonding, cutting, and/or rolling-up or folding-up enable effective production processes.

Furthermore, the support structure can be applied to the filter medium in a chemical melting method. Moreover, physical melting methods, especially adhesion methods, are conceivable for joining the support structure and the filter medium. Polyvinyl silicones are especially preferably used to form the plastic lattice and the support structure.

Other advantages and features of the invention will become apparent from the description and figures of the drawings. The aforementioned features and those presented further below can be implemented according to the invention individually or in any combination. The features shown in the figures are to be taken purely schematically and not to scale.

FIGS. 1 to 4 each show an enlarged plan view of one surface section of one exemplary embodiment of the filter material according to the invention.

FIG. 1 shows a rectangular section of a filter material 2 which has a flat shape and several layers of a filter medium 4 and a support structure 6. The filter medium 4 is produced from a plastic nonwoven in one layer. The support structure 6 comprises thread elements 10 a, 10 b, 10 c, 10 d, 10 e which run in a warp direction 8 and other thread elements 14 a, 14 b, 14 c which run in a weft direction 12. The thread elements 10 a, 10 b, . . . , 14 a, 14 b, . . . form a regular woven fabric, in the manner of a so-called plain weave, with the thread elements 10 a, 10 b, . . . , 14 a, 14 b, . . . each being woven in alternation at crossing sites over and then again under the next thread element 10 a, 10 b, . . . , 14 a, 14 b, . . . The thread elements 10 a, 10 b, . . . , 14 a, 14 b, . . . have essentially the same thickness d_(S), d_(K), The values for d_(K) are, for example, 246 μm and 262.4 μm; the values for d_(S) are, for example, 252.6 μm and 242.8 μm. Moreover, the thread elements 10 a, 10 b, . . . , and 14 a, 14 b, . . . which run in groups parallel to one another are arranged equidistant to one another, yielding a uniform mesh size of the passage openings 16 a, 16 b, 16 c, 16 d, 16 e, . . . which are arranged between the thread elements 10 a, 10 b, . . . , 14 a, 14 b, . . . . The mesh size of the woven fabric which forms the support structure 6 is designated L_(S)×L_(K) and is, for example, 869.3 μm×1243.3 μm for the passage opening 16 b; 862.8 μm×1217.1 μm for the passage opening 16 c; 908.7 μm×1233.5 μm for the passage opening 16 d.

The material of the support structure 6 is PBT plastic which has been applied to the filter medium 4 made as a polyester nonwoven in a coating process. Here adhesive has been applied or sprayed onto the filter medium 4 first in a spray application so that droplet-shaped adhesive spots 18, 18′, 18″, . . . of different sizes form and adhere to the filter medium 4 in an irregular arrangement. Then the support structure 6 which has been made first as a separate material layer is placed or pressed onto the filter medium 4, as a result of which on the adhesive spots 18, 18′, 18″, . . . which lie between the thread elements 10 a, 10 b, . . . , 14 a, 14 b, . . . and the nonwoven threads 20, 20′, 20″ . . . of the filter medium 4, an adhesive bond is formed and the multilayer filter material 2 is completed. The spray application of the adhesive yields an adhesive bond which is uniformly strong over the area of the filter medium 4 between the support structure 6 and the filter medium 4, as well as additional stiffening of the filter medium 4 produced from polyester nonwoven with the minimum possible adverse effect on the fluid permeability of the passage openings 16 a, 16 b, . . . occupied by the filter medium 4.

The filter material 2 shown in FIG. 2 differs from the one shown in FIG. 1 in that the filter medium 4 [has] an additional coating 22 by which the nonwoven threads 20, 20′, 20″ of the filter medium 4, or more precisely their intermediate spaces, are for the most part closed. The coating 22 is shown by way of example in a passage region between the thread elements 10 c, 10 d, 14 a, 14 b, but extends over the entire filter medium 4. In this way, the fluid permeability of the filter medium 4 is reduced, and consequently its filtration capacity is improved. The adhesive spots 18, 18′, 18″, . . . which have been applied in a spray application can likewise be easily recognized. The filter material 2 shown in FIG. 2 is used especially for fine filter elements. The thread elements 10 a, 10 b, . . . , 14 a, 14 b, . . . of the fabric-like support structure 6 run in groups parallel to one another and cross at a right angle. Parallelogram-like, especially diamond-shaped configurations of the fabric-like support structure 6 are, however, also conceivable.

FIG. 3 shows one exemplary embodiment with an electrically conductive thread 24 which is made as a carbon filament and which is woven into the fabric-like support structure 6. The electrically conductive threads 24 improve the electrostatic properties of the filter material 2 and reinforce it mechanically. The woven-in conductive thread 24 can run in the weft direction 12, as shown in FIG. 3, but an arrangement in the warp direction 8 is also conceivable. As shown in FIG. 3, it can replace a regular thread element 14 c, but can also in addition be worked in. In particular, other electrically conductive threads (not shown) can be worked into the support structure 6.

While in the exemplary embodiments shown in FIGS. 1 to 3 the thickness of the thread elements 10 a, 10 b, . . . , 14 a, 14 b, . . . is essentially the same and is about 250 μm, FIG. 4 shows a filter material 2 with thread elements 10 a, 10 b, . . . , 14 a, 14 b, . . . of different thicknesses. The thread elements 10 a, 10 b, . . . which run in the warp direction 8 have a thickness d_(K) of about 250 μm, such as 262.4 μm, 252.6 μm, and 259.2 μm. The other thread elements 14 a, 14, . . . which run in the weft direction 12 have a diameter and a thickness d_(S) of about 200 μm, such as 193.6 μm and 200.1 μm. This yields larger passage openings 16 a, 16 b, . . . and consequently improved fluid permeability of the support structure 6 and of the entire filter material 2. 

1. A filter material (2) for fluids, in particular for hydraulic fluids, comprising a filter medium (4) having at least one layer and a support structure (6) which is formed at least partially from a plastic material and which adjoins at least one side of the filter medium (4) in surface regions, characterized in that the filter medium (4) and the support structure (6) are joined to one another by means of lamination, coating, and/or by means of a melting method.
 2. The filter material according to claim 1, characterized in that adhesive spots (18-18″) which have been applied at least in individual areas in a spray application, especially in different sizes and/or in an irregular arrangement, adhere to the filter medium (4).
 3. The filter material according to claim 1, characterized in that the adhesive spots (18-18″) and/or an additional coating (22) provided on the filter medium (4) partially enclose or cover at least some of the passage openings (16 a-16 f) for fluid which are dictated by the support structure (6).
 4. The filter material according to claim 1, characterized in that the material of the support structure (6) comprises polybutylene terephthalate (PBT) plastic, polypropylene (PP) plastic, and/or polyethylene (PE) plastic.
 5. The filter material according to claim 1, characterized in that the support structure (6) is made as a lattice or a woven fabric.
 6. The filter material according to claim 5, characterized in that the thickness of thread elements (10 a, 10 b, . . . , 14 a, 14 b, . . . ) which form the lattice or the woven fabric varies over the area of the filter material (2), especially with the thread elements in the weft direction (14 a, 14 b, . . . ) having a thickness (dS) that is different from the thread elements in the warp direction (10 a, 10 b, . . . ).
 7. The filter material according to claim 6, characterized in that the thickness (dS, dK) of the thread elements in the weft direction is 250 □m (dS) and in the warp direction 200 □m (dK).
 8. The filter material according to claim 5, characterized in that the mesh size (LS×LK) of the lattice or of the woven fabric is 850 □m×1200 □m.
 9. The filter material according to claim 1, characterized in that the filter medium (4) is made as a nonwoven, preferably as a plastic nonwoven, and especially preferably as a polyester nonwoven.
 10. The filter material according to claim 1, characterized in that the material of the filter medium (4) comprises polybutylene terephthalate (PBT) plastic, polypropylene (PP) plastic, and/or polyethylene (PE) plastic.
 11. The filter material according to claim 1, characterized in that the filter medium (4) and the support structure (6) are joined to one another like a blanket.
 12. A method for producing a filter material (2) for fluids, in particular for hydraulic fluids, comprising a filter medium (4) which has at least one layer on at least one side being joined flat to a support structure (6) which is formed partially from a plastic material in surface regions, characterized in that the filter medium (4) and the support structure (6) are joined to one another by means of lamination, coating, and/or by means of a melting method.
 13. The method according to claim 12, characterized in that the support structure (6) is calender-coated onto the filter medium (4) as a coating.
 14. The method according to claim 13, characterized in that the support structure (6) is applied to the filter medium (4) in an immersion method.
 15. The method according to claim 13, characterized in that the support structure (6) is applied to the filter medium (4) in a doctoring method.
 16. The method according to claim 12, characterized in that the support structure (6) is calender-coated onto the filter medium (4) in an adhesive method.
 17. The method according to claim 16, characterized in that the adhesive (18, 18′, . . . ) is applied to the filter medium (5) in a carrier application.
 18. The method according to claim 16, characterized in that the adhesive (18, 18′, . . . ) is applied to the filter medium in a spray application.
 19. The method according to claim 18, characterized in that droplet-shaped adhesive spots (18-18″), especially of different sizes and/or irregular arrangement, are applied to the filter medium (4).
 20. The method according to claim 16, characterized in that an adhesive layer is applied to the filter medium (4) and/or the support structure (6).
 21. The method according to claim 16, characterized in that an adhesive (18, 18′, . . . ), especially a hot-melt adhesive, which self-adherently joins the filter medium (4) to the support structure (6), is used.
 22. The method according to claim 12, characterized in that the support structure (6) is applied to the filter medium (4) in a thermal melting method, especially in an ultrasonic method.
 23. The method according to claim 12, characterized in that the support structure (6) is applied to the filter medium (4) in a chemical melting method.
 24. The method according to claim 12, characterized in that the support structure (6) is applied to the filter medium (4) in a physical melting method, especially an adhesion method.
 25. The method according to claim 12, characterized in that an additional coating (22) is applied to the filter medium (4).
 26. The method according to claim 19, characterized in that the adhesive spots (18-18″) and/or the coating (22) are applied to the filter medium (4) such that at least some of the passage openings (16 a-16 f) for fluid which are dictated by the support structure (6) are partially enclosed or covered. 